NOAA is predicting a dead zone in the Gulf of Mexico along the coasts of Louisiana and Texas this summer larger than any on record, based on the amount of pollution coming down the Mississippi River this spring.

The dead zone has been in existence since the 1970s. It is caused when farm fertilizer and animal waste, washed into the river and flow downstream into the Gulf.

The nutrients feed algae that overgrow, and use up oxygen in the water at the bottom of the Gulf. Starved for oxygen, marine animals either move or die.